Windmills are not new. From at least the end of the dark ages in Europe, the wind has been employed in certain countries, notably Holland, to perform useful functions such as the pumping of water for purposes of clearing flooded or floodable land. These early windmills traditionally have been possessed of a certain timelessness in design, and have been captured by some of the great masters in paintings hanging in the finest museums.
While such early windmills typically were directly coupled to a power consuming device such as a water pump, more recently windmills have been coupled to devices configured for generating power in a transmittable form, such as electricity. The basic configuration of windmills has changed over time also. Gone for the most part are the quaint buildings housing the works of a windmill such as captured the fancy of certain great painters of the past. In their place stand spidery towers supporting aerodynamically efficient blades. Often the blades are capable of being positioned with respect to the towers so that as the direction from which the wind approaches the tower changes, the blades may be oriented to face the approaching wind at an angle most favorable to the extraction of energy from the approaching wind.
One factor appears to have remained dominant in the evolution of successful windmill designs, however. That factor would appear to be the orientation of the axis of rotation for the blades of the windmill. With few exceptions, this axis generally uniformly parallels the direction of wind motion across the windmill. As this direction of wind motion typically is also generally parallel to the surface of the Earth, these wind machines have become known as horizontal axis windmills as the axis of rotation of the blades most typically is horizontal to the surface of the Earth.
The use of a vertical axis in the construction of windmills has been attempted, but with the possible exception of the so-called Darrieus type windmills and their derivatives, these vertical axis windmills have provided generally less than satisfactory commercial performance. And Darrieus type windmills traditionally require a substantial vertical structure to support the windmill. Thus these vertical axis windmills have been characterized by massive vertical structures and substantial height requirements for use.
A general, survey-type reference on windmill designs may be found in a report prepared for the National Science Foundation by Frank R. Eldridge of the Mitre Corporation under Grant No. AER-75-12937 in 1975. This report may be obtained from the Superintendent of Documents, United States Government Printing Office under stock number 038-000-00272-4.
While one prime focus in the use of windmills has been upon the generation of electrical power, other uses are possible where the windmill can be of a size, configuration and orientation suitable for application to a particular use. For example, in modern buildings with high ceilings it is often useful to employ ceiling fans to assist in the movement of air about the building to reduce the stratification of air into thermal layers within the building and the accompanying inefficiencies in heating and cooling. While these ceiling fans are most typically powered by electrical motors, they could as easily be powered by a rooftop mounted windmill having a rotational shaft configured to penetrate the roof to drive the fan.
But many building owners may object to the presence of typical tall spidery horizontal axis windmill structures upon their building as being at the least ungraceful, as well as likely imposing difficult structural loading problems in supporting these towers against the forces applied thereagainst by the movement of wind during violent storms. While blades characterizing many windmill designs may be feathered to reduce resistance to wind passage, the tower structure supporting the blades in many windmills poses a wind resistance which cannot easily be feathered.
Accordingly, a vertical axis windmill having a desirably low profile and yet an efficient mode of operation that could be used upon rooftops without unduly detracting from the appearance of the building embodying the windmill could find substantial use in commerce. Where such a windmill can generate additionally a sufficient torque to drive an electrical generating device, such windmills could have substantially enhanced utility in commerce.
During periods of violent weather windmills are vulnerable to damage due to the excessive forces imposed upon the windmill blades by the violent weather. Various proposals have been put forward for reducing this tendency for damage. Some have provided means for feathering the blades; others have provided means for governing the ultimate speed of rotation for the windmill or have allowed free rotation of the windmill blades while limiting or discontinuing power transfer to power consuming devices attached to the windmill during times that wind velocities are excessive to avoid thereby damage to power consuming devices. Such devices in the past have tended to be somewhat bulky, prone to less than reliable operation or incapable of protecting the windmill itself from damage from excessive wind velocity.
A windmill having a self governing feature to limit rotational velocity or power transfer during violent weather could find considerable application in commerce.